Around 400 million years ago, in the Silurian Era, the first plants
appeared on land. Most similar to what are known today as Bryophytes, they
descended from early water dwelling alga. They lacked vascular (circulatory)
systems and complex physical characteristics, but their appearance marked
a great step in the development of Earth. The world they populated was
far different from that which we enjoy today: rocky, exposed and barren,
with no sign of the rich and diverse life which was to later make our planet
so unique. Only the oceans, from which the first plants came, teemed with
organisms.
As time passed, the environment caused the first plants to change and
modify their structures to be able to meet their life needs. Further impetus
for evolution arrived with the first terrestrial animals, as the first
links in the great chain of animal-plant codependence were forged. With
change came specialization and increasing complexity in structure, and
it is based upon these characteristics that plants are classified or "ranked"
today in the study of phylogenetics. Classification begins with comparing
the primitive features of the earliest terrestrial plants to those of other
plants. In this method, we can organize terrestrial plants into the following
basic divisions, in order of increasing development: Bryophyta, Pteridophyta
(ferns and fern allies), gymnosperms (Cycadophyta, Ginkgophyta, Coniferophyta,
and Gnetophyta), and angiosperms (Anthophyta, divided into dicots and monocots).
In this tour, we will explore the evolution of the unique kingdom Plantae's
terrestrial members. To make matters simpler, we will focus mainly on comparing
the reproductive characteristics of each group. Other features also serve
to illustrate the evolutionary relationships, but the reproductive differences
are vivid and clear markers of the progression of terrestrial plant advancement.
As you proceed, consider the increasing complexity of the plants as we
progress from the most primitive Bryophytes to the most modern angiosperms.
Most of all, enjoy yourself as you delve into the wonderful and fascinating
world of plants.

Bryophytes

Bryophytes, the mosses, liverworts and hornworts, are the most primitive
terrestrial plants surviving today. Many fossils of plants long extinct
bear features similar to the modern Bryophytes, particularly, their lack
of vascular systems. Based upon genetic analysis, it seems that liverworts
are the most primitive of the three kinds, and mosses the closest relatives
to vascular plants.
Bryophytes are the second largest group of land plants in the
world today with over 20,000 species. Like their close relatives, the algae,
they require very moist conditions for survival, and their life processes,
especially reproduction, rely on water for their very execution.

Hornwortphoto courtesy U. of Wisconsin

Liverwort (Marchantia sp.)
photo courtesy U. of Wisconsin

Sphagnum Mossphoto courtesy U. of Wisconsin

Moss Life Cycle

Reproduction in the Bryophytes is the simplest of all terrestrial plants.
In mosses, it begins with the production of spores. The spores, manufactured
by mature sporophytes, form in capsules at the tops of long stemlike
stalks called seta. The capsules are protected by the capiltera.
Spores are released when the capsule's lid, or operculum, ruptures. When
moistened and in proper conditions, these spores germinate to form protonemas,
long fiberlike structures which branch. These protonemas produce gametophytes,
small leafy plants, male and female. It is these gametophytes, the leafy
plants, which we typically associate with mosses. Sperm is produced within
the antheridium of the male gametophyte, eggs are found in the female gametophyte's
archaegonium. Water transports mature sperm released by the antheridium
to the archaegonium, where fertilization occurs. Fertilization produces
a zygote, which grows into an embryo and eventually a mature sporophyte,
with seta, calyptra and capsule (sporangium) complete for a new cycle of
reproduction.

photos courtesy U. of Wisconsin

Ferns and Fern
Allies

Ferns and their allies are the earliest vascular plants. Grouped with
the ferns are other primitive vascular plants, among them the horsetails,
club mosses and whisk ferns. The fern group is far smaller now than it
once was. During the Carboniferous Period, Pteridophytes dominated the
plant world. Many of the early ferns grew to immense, treelike sizes, and,
when they died, set down layers of organic matter which were to become
the coal deposits of today.

Young fern fronds like the two in this picture are are nicknamed
fiddleheads, because they resemble the curved head of a violin. As they
mature, they unwind into the mature fronds with which most people are familiar.

Whisk Fern
Isoetes sp.photo courtesy U. of Wisconsin

Horesetail
Equisetum sp.

Club Moss
Selaginella sp.

Ferns Come in Many Shapes and Sizes

Aspelnium nidusBird's Nest Fern

Platycerium sp.
Staghorn Fern

Neprolepsis sp.

Fern Life Cycle

Like the Bryophytes from which they descend, Pteridophytes require moist
conditions for survival and reproduction. The principle element of their
reproductive cycle is the spore. The spores are produced in sacs called
sporangia and held in clusters called sori on the leaf surfaces, usually
the undersides. These spores, when mature, are released, and, if the surface
they find is moist, they will germinate.
The germinated spore, called the prothallus, is usually heart
shaped and photosynthetic. It has shallow rhizoids on its lower surface
for water absorption. When the prothallus is mature, it produces,
in two separate regions, sperm and egg cells. Sperm is released when the
prothallus comes in contact with water, often rain. These sperm are attracted
by chemicals secreted by the eggs. The eggs are fertilized by the sperm
and become zygotes. Over time, as their cells divide and multiply,
the zygote becomes embryos. The prothallus remains for some time,
eventually withering away as the zygote, now called an embryo, grows. The
embryo continues to experience growth until it is a mature adult fern (sporophyte)
and capable of reproducing, thus beginning the cycle again.

photos and drawings courtesy U. of Wisconsin

Gymnosperms

Gymnosperms are a familiar group nearly everywhere on earth. Ranging
from the exotic, ancient looking cycads to the towering Sequoias, they
area very diverse and widespread group. Today, we use them for everything
from Christmas trees to medicinal Gingko extracts to the special flavoring
of gin. Within the group, the cycads, or seed ferns, are most likely to
have evolved first, and the Gnetophytes, which many consider to be a missing
link between gymnosperms and angiosperms, most recently.
The evolution of the gymnosperms brought an entirely new concept
to plant reproduction: seeds. Unlike the ferns and bryophytes before them,
gymnosperms had no need for extremely moist conditions for reproduction.
Instead, embryonic plants were neatly packages for dispersal and germination
in favorable conditions. In other words, gymnosperms rocked the plant world.

a closeup of the seeds in a cycad coneZamia sp.

cycad

coniferAraucaria heterophylla

gnetumGnetum gnemon

Gingko biloba

Pine Life Cycle

Unlike ferns, gymnosperms produce their gametophytes (sperm and eggs) in
different, more specialized areas. As they are most familiar, we will consider
the life cycle of a pine tree. As an adult, mature tree, the pine produces
two different types of cones, pollen cones (male) and ovulate cones (female).
The male gametophytes exist within special sacs as pollen grains, while
the female gametophytes, the eggs, are held within ovules, protected by
a layer called the integument. The integument has a single opening, called
the micropyle. An ovule may hold one or more eggs. When
the pollen grains are mature they are released and borne by the wind to
the ovules' micropyles. If they are accepted by the ovule, the pollen grains
germinate and produce a tube leading into the integument. The sperm, held
within the pollen grains, pass down these tubes and meet the eggs, which
they fertilize. When the eggs are fertilized, the integument thickens into
a seed coat, to protect the embryonic plant within. The seed also has nutritive
tissues which supply food for the embryo until the seed can germinate and
the plant can produce food for itself through photosynthesis. The seed,
when developed and mature, is released in a variety of ways, including
wind dispersal. If it finds suitable growing conditions where it lands,
the seed will germinate and develop into a new plant, capable on maturity
of producing more seeds and continuing the cycle.

photos courtesy U. of Wisconsin

Angiosperms

Think of how different our world would be if flowering plants had never
evolved. Imagine a spring without those lovely monocots, the flowering
bulbs. Or an anniversary sans a dozen dicots; roses. There
would be no fruit, no grains, no need for pollinating insects, no perfumes,
and few textile sources. This short segment of a long list is ample
illustration of the vast importance of angiosperms in our lives.
The flower is the main reproductive center of the angiosperm.
Some angiosperms produce only one type of gametophyte, either male or female,
per plant. They are thus termed dioecious, and these plants are either
male or female. An example of this type of plant is holly. Others, which
are monoecious, produce male and female gametophytes on different flowers
of the same plant. An example of this is corn (Zea mays). The last type
has both male and female parts in a single flower.

Flower Parts

Monocots vs. Dicots

Angiosperms are divided into two main groups: monocots and the
more primitive dicots. You can see their differences illustrated on the
chart at this station. The dicot magnolia family is considered to be the
oldest group of angiosperms, the monocot orchids the most recent.

flower parts in 4's and 5's

leaves with "netted" veins

bean seed
photo courtesy U. of Wisconsinseeds with two cotyledonsThe peanut is another example of a dicot seed

flower parts in 3's

leaves with veins that run parallel to leaf edges (margins)

corn kernel
photo courtesy U. of Wisconsin

seeds have one cotyledon

Angiosperm Life Cycle

The male gametophytes, called microgametophytes, are produced in anther
sacs in the anthers of the flower's stamens. Like the male gametophytes
of the gymnosperms, the sperm is encased in a protective cover and this
complex is called a pollen grain. As the microgametophyte matures, this
cover diminishes, leaving behind a two celled male gametophyte.
The female gametophytes, megagametophytes, are produced in the
ovary of the flower. The ovary is made up of one or many carpels, either
fused or unfused. Each carpel contains ovules, immature seeds, singly
or in multitudes, depending on the type of plant. The ovary is part of
a larger structure, collectively called the gynoecium, which makes up the
entire female reproductive system. Aside from the ovary, it consists of
a stigma, a sticky surface where pollen grains adhere, and a pistil, a
long tube leading from the stigma to the ovary's inside.
When pollen is mature, it is released for dispersal. Some of
it eventually, through various means, reaches the stigma, where it adheres.
One of the two cells of the pollen grain begins to work its way down the
inside of the pistil to the ovary, forming a pollen tube. The other cell,
the sperm, or generative cell, follows. When the generative cell reaches
the ovule, it undergoes a series of steps and eventually the ovule is fertilized.
The fertilized ovule is now seed containing a developing zygote. The zygote
grows and differentiates into an embryo, nourished by stored nutrients
in the seed's endosperm. When the seed is mature, it is released through
various means. Should it find a fertile growing location, it will germinate
and grow to produce a mature plant which in turn can carry on the cycle.